Modern hybrid vehicles may use either various forms of energy in addition to combustion engines for additional power and energy storage. The most well know hybrid cars are electronic hybrids that incorporate batteries which store electrical energy. Energy wasted during braking may be converted into electrical power and stored in these batteries, which then may be used to when the hybrid car is accelerating. Such hybrids are commercially available and generally get better gas mileage than cars of generally the same weight and horsepower.
Also known in the patent art are hydraulic hybrids which operate on a similar concept to the electrical hybrids. An example of a hydraulic hybrid is disclosed in U.S. Pat. No. 4,098,083 to Carman, which is herein incorporated by reference for all that it contains. The patent discloses a hydraulic multi-speed, multi-torque transmission system for vehicles for storing and converting energy resulting from braking of the vehicle, such transmission utilizing multiple fixed displacement hydraulic pump-motors coupled to the vehicle wheels and a fixed displacement pump driven by the engine. Carman discloses that the accumulators are used to store hydraulic energy. These accumulators use compressible gas to store the potential energy of the hydraulic system and to force to the hydraulic fluid into pump-motors.
Another type of hybrid described in the patent art is disclosed in U.S. Pat. No. 4,479,356 to Gill, which is herein incorporated by reference for all that it contains. It discloses an energy recovery system for a machine, and particularly an automotive vehicle, which includes an energy storage device which selectively receives energy from the prime mover for storage via a predetermined path and delivers stored energy to the prime mover load via the same path. In the preferred embodiment the energy storage device is an elastomeric tube which is disposed for rotational movement about a shaft and secured at the ends to respective end members. Braking units permit selective braking and releasing of the end members relative to fixed supports. A set of planet gears are controlled by the brake actuation to drive a sun gear to be driven by a ring gear to effect energy inflow and outflow, respectively, from the elastomeric tube. The storage device is permitted to store energy or release its stored energy as a function of vehicle control and operational parameters.